Shot peening method and device therefor

ABSTRACT

The present invention provides a method for a shot-peening processing that can process in one step and thus reduce operating costs and the cost of equipment while increasing the efficiency of the treatment in the peening processing while achieving a peening effect similar to the effect obtained by shot-peening processing a plural number of times, that is, deeply generating compressive residual stress far from the top surface while generating the peak value of the compressive residual stress on the top surface. This method is characterized by projecting, on a product to be subjected to peening, shot in which two or three kinds of particles, each having a different predetermined average particle diameter and having a predetermined average particle diameter ratio to one another, are combined in a predetermined weight ratio.

FIELD OF THE INVENTION

This invention relates to a method and device for effectivelyshot-peening curburized products such as gears, etc.

BACKGROUND OF THE INVENTION

Generally known has been a shot-peening processing in which particulateshot are projected on the surface of the part that receives theconcentration of stress of the curburized products such as gears, etc.,to increase the compressive residual stress and improve the fatiguestrength. Japanese Patent Early-publication Nos. 60-150966 and 61-265271suggest that a double shot-peening processing be conducted to increasethe effect of a shot-peening processing.

In the double shot peening, at the first stage a deep (thick)compressive residual stress layer is formed in a product (the brokenline in the graph of FIG. 2) by applying to a product a shot-peeningprocessing with large-diameter particles, and at the second stage a highcompressive residual stress is obtained in the top of the surface layerin the product by applying to the product to a shot-peening processingwith small-diameter particles (the thick, continuous line in the graphof FIG. 2). By applying such a double shot-peening processing, acompressive residual stress as shown by the broken line in the graph ofFIG. 1 is obtained.

However, since the above double shot-peening processing has twoprocessing steps, it is time-consuming and thus the efficiency ofprocessing worsens. Also, it requires equipment of two devices, one ofwhich is used for projecting the large-diameter particles and the otherof which is used for projecting the small-diameter particles. Thus ithas a problem, in that the cost of equipment is high.

On the other hand, Japanese Patent Early-publication No. 60-96717discloses an invention in which a shot peening is applied to the surfaceof a spring to generate compressive residual stress. Then a honing orsandblasting is applied to the surface to improve the surface roughnessand increase the fatigue strength of the spring. However, this processalso requires equipment of two devices. Thus the cost of equipment ishigh.

DISCLOSURE OF THE INVENTION

The present invention aims at providing a method and device for ashot-peening processing that can reduce operating costs and the cost ofequipment while increasing the efficiency of treatment in the peeningprocessing while achieving in a peening effect similar to the effectobtained by shot-peening processing plural times, that is, deeplygenerating compressive residual stress far from the top of the surface,while retaining the generation of the peak value of the compressiveresidual stress on the top of the surface.

The inventors of the present application extensively investigated toachieve the above object. As a result, they have found a method forshot-peening processing using shot in which two or three kinds ofparticles, each having an average particle diameter within the range ofthe predetermined average particle diameters, the ranges differing fromone. another, and having a predetermined average particle diameter ratioto one another, are combined in a predetermined weight ratio, and adevice for conducting this method.

Method 1 of the present invention is characterized by projecting, on aproduct to be subjected to peening, shot in which large-diameterparticles having an average particle diameter of 300-1,000 μm, andsmall-diameter particles having an average particle diameter of 20-300μm, the ratio of the average particle diameter of said small-diameterparticles to that of said large-diameter particles being 1/3-1/15, arecombined in a weight ratio such that the coverage of each of theparticles is 100% or more in the same projection time.

Method 2 of the present invention is characterized by projecting, on aproduct to be subjected to peening, shot in which large-diameterparticles, having an average particle diameter of 500-1,000 μm,medium-diameter particles having an average particle diameter of 100-500μm, and small-diameter particles having an average particle diameter of20-100 μm, the ratio of the average particle diameter of saidmedium-diameter particles to that of said large-diameter particles andthe ratio of the average particle diameter of said small-diameterparticles to that of said medium-diameter particles each being 1/2-1/15,are combined in a ratio such that the coverage of each of the particlesis 100% or more in the same projection time.

Also, the first device for a shot-peening processing to conduct themethod of the present invention is characterized by connectively placinga classifying device at the bottom of a peening chamber provided with aprojecting device at its top, by which classifying device shot that areworn and broken by their use into particles having a particle diameterother than the predetermined average particle diameters of the particlesto be used are classified and removed while recycled shot are classifiedinto particles of respective average particle diameters to be used (forabove method 1, the large-diameter particles and the small-diameterparticles, and for above method 2, the large-diameter particles, themedium-diameter particles, and the small-diameter particles),connectively joining the respective openings of the classifying devicefor discharging the large-diameter particles and the small-diameterparticles to respective means for transferring shot, connecting the endsof the respective means for transferring shot to a tank for shot, inwhich tank a device for uniformly stirring and mixing is provided, andconnectively joining the tank to the projecting device.

A second device for a shot-peening processing to conduct the method ofthe present invention is characterized by connectively placing aclassifying device at the bottom of a peening chamber provided with aprojecting device at its top, by which classifying device shot that areworn and broken by their use are classified into particles havingparticle diameters other than predetermined average particle diametersof the particles to be used and removed while recycled shot areclassified into particles of respective average particle diameters to beused (for above method 1, the large-diameter particles and thesmall-diameter particles, and for above method 2, the large-diameterparticles, the medium-diameter particles, and the small-diameterparticles), connectively joining the respective openings of theclassifying device for discharging particles of respective averageparticle diameters (for above method 1, the large-diameter particles andthe small-diameter particles, and for above method 2, the large-diameterparticles, the medium-diameter particles, and the small-diameterparticles) to respective means for transferring shot, collectivelyconnecting the ends of the respective means for transferring shot torespective supply pipes for the shot through respective flow controlvalves by which each flow rate is controlled to be at a predeterminedweight ratio, and connectively joining the supply pipes to theprojecting device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph representing the states of residual stress generatedby conducting method 1 of the present invention and of the residualstress generated by the double peening processing.

FIG. 2 is a graph representing a state of the residual stress generatedby conducting respective peening processings with the large-diameterparticles and the small-diameter particles, both types of said particlesof respective particle diameters being used in method 1 of the presentinvention.

FIG. 3 is a graph representing the states of the residual stressgenerated by conducting method 2 of the present invention and of theresidual stress generated by the three-step peening processing.

FIG. 4 is a graph representing a state of residual stress generated byconducting a respective peening processing with the large-diameterparticles, the medium-diameter particles, and the small-diameterparticles, all types of the particles of said average particle diametersbeing used in method 2 of the present invention.

FIG. 5 is a graph representing an example of determining a coverage in apeening processing with the large-diameter particles and thesmall-diameter particles, both types of the particles of said averageparticle diameters being used in method 1 of the present invention.

FIG. 6 is a constitutional drawing showing a first shot-peeningprocessing device to conduct method 1 of the present invention.

FIG. 7 is a constitutional drawing showing a second shot-peeningprocessing to conduct method 1 of the present invention.

FIG. 8 is a constitutional drawing showing a first shot-peeningprocessing device to conduct method 2 of the present invention.

FIG. 9 is a constitutional drawing showing a second shot-peeningprocessing device to conduct method 2 of the present invention.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

By projecting, on a product to be subjected to peening, shot in whichlarge-diameter particles having an average particle diameter of300-1,000 μm, and small-diameter particles having an average particlediameter of 20-300 μm, the ratio of the average particle diameter ofsaid small-diameter particles to that of said large-diameter particlesbeing 1/3-1/15, are combined in a weight ratio such that the coverage ofeach of the particles is 100% or more in the same projection time(method 1), peening processes can be conducted in one step while apeening effect is obtained equivalent to that obtained by a doublepeening processing in which the large-diameter particles and thesmall-diameter particles are separately projected. Thus this method cansignificantly increase the efficiency of treatment. Also, in this Amethod the equipment for peening processing can be that for conducting aone-step peening processing. Thus the cost of equipment can besignificantly decreased.

By projecting, on a product to be subjected to peening, shot in whichthe large-diameter particles having an average particle diameter of500-1,000 μm, the medium-diameter particles having an average particlediameter of 100-500 μm, and the small-diameter particles having anaverage particle diameter of 20 -100 μm, the ratio of the averageparticle diameter of said medium-diameter particles to that of saidlarge-diameter particles and the ratio of the average particle diameterof said small-diameter particles to that of said medium-diameterparticles being each 1/2-1/15, are combined in a ratio such that thecoverage of each of the particles is 100% or more in the same projectiontime (method 2), the peening process can be conducted in one step, whilea peening effect is obtained equivalent to that obtained by a triplepeening processing in which the large-diameter particles, themedium-diameter particles, and the small-diameter particles areseparately projected. That is, compressive residual stress is deeplygenerated far from the top of the surface, and the peak value of thecompressive residual stress is generated on the top of the surface. Thusthis method can significantly increase the efficiency of the treatment.Also, the equipment for peening processing can be that for conducting aone-step peening processing. Thus the cost of equipment can besignificantly decreased.

From the results of various experiments, it was found that since the useof shot having an average particle diameter of less than 20 μm for thesmall-diameter particles has only a slight effect in peening processing,and the use of shot having an average particle diameter of more than1,000 μm for large-diameter particles makes the surface roughness ofproducts to be processed high and fatigue strength significantly low,average particle diameters of 20-1,000 μm are preferred for the scope ofthe application of the shot of the present invention.

For the method of determining the strength of shot-peening processing,the method of measuring area coverage (hereafter, “coverage”) isgenerally utilized. This measurement of the coverage is obtained bycalculating the ratio of the total of the areas of traces caused by theprojection with shot to a processed area. For measuring the areacoverage, there is a standard measurement method and a simplifiedmeasurement method. For the experiments of the present application itwas determined by the simplified measurement method. This simplifiedmeasurement method is one that determines a coverage by comparing aphotograph of a standard of which the coverage is already known by thestandard measurement method with the surface of a specimen afterprocessing. In FIG. 5 an example is shown of measuring the coverage bymethod 1 of the present invention. In that Figure the continuous linerepresents the characteristics of the coverage of the large-diameterparticles and the broken line represents the characteristics of thecoverage of the small-diameter particles. When the coverage by one-timepeening is C₁, then C_(n)=1−(1−C₁) “n” where “n” is the number of timesof peening. The calculated value is to an extent of 98%, which isregarded as full coverage, and as 100%. Also, 300% of coverage isreferred to as being in the state in which the time until the coveragereaches 100% is multiplied by three.

A first device for conducting a method of projecting, on a product to besubjected to peening, shot in which the large-diameter particles and thesmall-diameter particles are combined in a predetermined weight ratio,each of said large-diameter particles and small-diameter particlessatisfying their respective requirements (method 1), is specificallyexplained below by referring to FIG. 6.

The bottom part of a peening chamber 2, to the top of which a projectingdevice 1 for shot is fixed, communicates with a classifying device 3through a recycle pipe. By the classifying device 3, shot that are wornor broken into particles having an average particle diameter other thanthat of large-diameter particles or the small-diameter particles(particles intermediate between the large-diameter particles and thesmall-diameter particles, and fine powder smaller than thesmall-diameter particles) can be classified and subsequently recycled inrecycle boxes 4A and 4B and removed while classifying the shot into thelarge-diameter particles and the small-diameter particles, which aresubsequently delivered.

Openings 5A and 5B for discharging the large-diameter particles and thesmall-diameter particles, respectively, which are classified by theclassifying device, communicate with a tank 8 for shot disposed abovethe projecting device 1 through a means 6 for transferring shot,constituted by a screw conveyor or bucket conveyor. In the tank 8 forshot a device 9 for a uniform stirring and mixing is provided. Also, thetank 8 for shot communicates with the projecting device 1.

In this device, shot in which the large-diameter particles and thesmall-diameter particles are combined in a predetermined weight ratioare supplied from the tank 8 for shot to the projecting device 1 bywhich the shot are projected toward a product W to be subjected topeening, which product W is revolved in the peening chamber 2 to applypeening processing. Since in the tank for shot they are retained suchthat the large-diameter particles and the small-diameter particles aresegregated by their specific gravities, the device 9 for a uniformstirring and mixing is operated to uniformly stir and mix the shot tobreak up the segregation and then to supply the materials to theprojecting device 1. Thus a peening processing is again conducted in astate such that satisfies predetermined processing conditions.

Next, similarly to the above, a device for a second shot-peeningprocessing to conduct method 1 is specifically explained below byreferring to FIG. 7.

A peening chamber 2, to the top of which a projecting device 1 is fixed,a classifying device 3, and recycle boxes 4A and 4B are composed asshown by FIG. 6. An opening 5A for discharging the large-diameterparticles and an opening 5B for discharging the small-diameter particlesof the classifying device 3 communicate with respective means 6A and 6B,respectively, for transferring shot, constituted by a screw conveyor orbucket conveyor. The end of each of the transferring means 6A and 6Bcommunicates with the projecting device 1 through respective flowcontrol valves 7A and 7B and respective supply pipes 10A and 10B forrespective shot.

In this device, the large-diameter particles and the small-diameterparticles are delivered in respective predetermined weights viarespective flow control valves 7A and 7B of the large-diameter particlesand the small-diameter particles to be controlled in a predeterminedweight ratio, and are supplied through supply pipes 10A and 10B for shotto a projecting device 1 by which the shot are projected toward aproduct W to be subjected to peening, which product W is revolved in thepeening chamber 2, to apply peening processing.

A first device for conducting a method of projecting, on a product to besubjected to peening, shot in which the large-diameter particles, themedium-diameter particles, and the small-diameter particles, each of theparticles of said diameters satisfying their respective requirements,are combined in a predetermined weight ratio (method 2), is specificallyexplained below by referring to FIG. 8.

A peening chamber 2, to the top of which a projecting device 1 is fixed,a classifying device 3, and recycle boxes 4A and 4B are composed. By theclassifying device 3, shot that are worn or broken by their use intoparticles having an average particle diameter other than that of thelarge-diameter particles, the medium-diameter particles, or thesmall-diameter particles (particles intermediate between thelarge-diameter particles and the small-diameter particles, particlesintermediate between the medium-diameter particles and thesmall-diameter particles, and fine powders smaller than thesmall-diameter particles) are classified and are recycled in recycleboxes 4A, 4B, and 4C and removed while the recycled shot are classifiedinto the large-diameter particles, the medium-diameter particles, andthe small-diameter particles, each of which has a predetermined averageparticle diameter. Discharge openings 5A, 5B, and 5C, for thelarge-diameter particles, the medium-diameter particles, and thesmall-diameter particles, respectively, which are classified by theclassifying device 3, communicate with a column of means 6 fortransferring shot, constituted by a screw conveyor or bucket conveyor.The end of the transferring means 6 communicates with a tank 8 for shotplaced above the projecting device. In the tank 8 for shot, a device 9for a uniform stirring and mixing is installed. The lower end of thetank for shot communicates with the projecting device 1.

In this device, shot in which the large-diameter particles, themedium-diameter particles, and the small-diameter particles are combinedin a predetermined weight ratio are supplied from the tank 8 for shot toa projecting device 1 by which the shot are projected toward a product Wto be subjected to peening, which is revolved in the peening chamber 2,to apply peening processing. Since in the tank 8 for shot the shot areretained such that they are segregated by their specific gravities, thedevice 9 for a uniform stirring and mixing is made to operate touniformly stir and mix the shot to break up the segregation. Then theshot are supplied to the projecting device 1. Thus peening processing isagain conducted in a state such that predetermined processing conditionsare satisfied.

Next, similarly to the above, a device for a second shot-peeningprocessing to conduct method 2 is specifically explained below byreferring to FIG. 9.

A peening chamber 2, to the top of which a projecting device 1 is fixed,a classifying device 3, and recycle boxes 4A, 4B, and 4C are composed asshown by FIG. 8. An opening 5A for discharging the large-diameterparticles, an opening 5B for discharging the medium-diameter particles,and an opening 5C for discharging small-diameter particles of theclassifying device 3 communicate with a shot-transferring means 6A, 6B,and 6C, respectively, for transferring shot, constituted by a screwconveyor or bucket conveyor. The end of each of the transferring means6A, 6B, and 6C communicates with the projecting device 1 throughrespective flow control valves 7A, 7B, and 7C and respective supplypipes 10A, 10B, and 10C for respective shot.

In this device, the large-diameter particles, the medium-diameterparticles, and the small-diameter particles are delivered in respectivepredetermined weights via respective flow control valves 7A, 7B, and 7Cfor the large-diameter particles, the medium-diameter particles, and thesmall-diameter particles and in a controlled predetermined weight ratio,and are supplied through the supply pipes 10A, 10B, and 10C for shot toa projecting device 1 by which the shot are projected toward a product Wto be subjected to peening, which is revolved in a peening chamber 2, toapply peening processing.

EXAMPLES

In the following Examples, as a product to be subjected to peening, acommercial gear of alloy steel to use for machine structure that hasbeen treated with carburization is used. This gear has a diameter of 130mm and a width of 15 mm, and a hardness of HV720-850.

Example 1

In this Example, an example in which the shot, in which thelarge-diameter particles and the small-diameter particles within therespective ranges of the predetermined average particle diameters werecombined, are projected on a product to be subjected to peening, i.e.,the gear (method 1 of the present invention), is given with a comparisonto the case in which a double shot-peening processing is conducted.

The conditions for the peening processing on this gear are as shown byTable 1. As the shot, large-diameter particles A (an average particlediameter of 600 μm) and small-diameter particles B (an average particlediameter of 100 μm) were used. They were projected at a rate of 70m/second for 28 seconds. For a double shot-peening, A was projected for18 seconds and B was projected for 10 seconds.

TABLE 1 Shot A: diameter: 600 μm B: diameter: 100 μm Amount projected170 kg/min. Projection time A: 18 sec. B: 10 sec. Double peening: 28sec. Mixed peening: 28 sec. Rate of projection 70 m/sec. Number ofrevolutions of product 60 rpm Coverage 300% or more

To equalize the projection energy to that given upon double shot-peeningprocessing, a peening processing was conducted at a weight ratio of thelarge-diameter particles A to the small-diameter particles B used inthis Example of 64.3 for A and 35.7 for B. Thus if the total projectiontime is 1, the ratio of A to B to be projected is 0.643 for A and 0.357for B. In the present case, the coverage was set to be 300%, so thateach of the conditions could be compared.

As shown by the thick continuous line in the graph of FIG. 1, in thegear processed with a shot-peening with method 1 of the presentinvention (using shot in which the large-diameter particles and thesmall diameter-particle were combined) the peak was observed of acompressive residual stress at the predetermined place in the directionof the depth from the surface of the product to be subjected to peening.Also, it was found that a compressive residual stress was generated to adepth of 100-150 μm from the surface (the thick continuous line in thegraph of FIG. 1). It was also found that the properties were almost thesame as those of the double shot-peening processed product (the brokenline in the graph of FIG. 1).

Example 2

In this Example, an example in which shot in which the large-diameterparticles, the medium-diameter particles and the small-diameterparticles having respective average particle diameters within therespective ranges of predetermined average particle diameters werecombined were projected on a product to be subjected to peening, i.e.,said gear (method 2 of the present invention), is given with acomparison to the case in which a three-step shot-peening processing wasconducted.

The conditions for the peening processing on this gear were as shown byTable 2. As the shot, large-diameter particles A (an average particlediameter of 800 μm), medium-diameter particles B (an average particlediameter of 250 μm), and small-diameter particles C (an average particlediameter of 40 μm) were used. They were projected at a rate of 70m/second for 48 seconds. For the three-step shot-peening, thelarge-diameter particles A were projected for 20 seconds, B wereprojected for 12 seconds, and C were projected for 8 seconds).

TABLE 2 Shot A: diameter: 800 μm B: diameter: 250 μm C: diameter: 40 μmAmount projected 170 kg/min. Projection time A: 20 sec. B: 12 sec. C: 8sec. Three-time peening: 40 sec. Mixed peening: 40 sec. Rate ofprojection 70 m/sec. Number of revolutions of product 60 rpm Coverage300% or more

When the total shot are regarded as 100, to equalize the projectionenergy to that given upon three-step shot-peening processing, ashot-peening processing was conducted with the large-diameter particlesA of 50, the medium-diameter particles B of 30, and the small-diameterparticles C of 20. Thus when the total projection time is regarded as 1,the weight ratio of the shot to be projected was 0.5 for A, 0.3 for B,and 0.2 for C. In the present case, the coverage was set to be 300%, sothat each of the conditions could be compared.

As shown by the thick continuous line in the graph of FIG. 3, in thegear processed by a shot-peening with method 2 of the present invention(using shot in which the large-diameter particles, the medium-diameterparticles, and the small-diameter particles were combined) the peak wasobserved of a compressive residual stress at almost the top of thesurface of the product subjected to peening. Also, it was found that acompressive residual stress was generated to 250-300 μm from thesurface. It was also found that the properties were almost the same asthose of the three-step shot-peening processed product (the broken linein the graph of FIG. 3).

What is claimed is:
 1. A method for shot-peening the surface of aproduct, comprising combining in a predetermined weight ratiolarge-diameter particles having an average particle diameter in therange of 300 to 1,000 μm and small-diameter particles having an averageparticle diameter in the range of 20 to 300 μm to form combinedparticulate shot, providing a ratio of the average particle diameter ofsaid small-diameter particles to the average particle diameter of saidlarge-diameter particles in the range of 1/3 to 1/15, shot-peening thesurface of the product by projecting the combined particulate shot uponthe surface of the product, and obtaining during the shot-peeningprocess by the predetermination of the weight ratio of large-diameterparticles to small-diameter particles an area of coverage of the surfaceof the product by each of the large-diameter particles andsmall-diameter particles of at least 100%, said shot-peening generatingcompressive residual stress at the surface of the product and in a deeplayer below the surface of the product.
 2. The method for shot-peeningas claimed in claim 1, further comprising recycling the combinedparticulate shot after the shot-peening step including removingparticulates that are worn and particulates that have been broken wherethe diameters are outside of said ranges of particle diameters of thelarge-diameter particles and the small-diameter particles, separatingthe residual shot into the large-diameter and the small-diameterparticles, combining again the large-diameter particles and thesmall-diameter particles to form combined particulate shot, uniformlymixing the combined particulate shot, and projecting again the combinedparticulate shot upon the surface of the product.
 3. The method forshot-peening as claimed in claim 2, further comprising rotating theproduct during the shot-peening step.
 4. The method for shot-peening asclaimed in claim 2, further comprising transporting the combinedparticulate shot to a tank where the mixing step occurs, and stirringwhile mixing the combined particulate shot.
 5. The method forshot-peening as claimed in claim 1, further comprising recycling thecombined particulate shot after the shot-peening step includingclassifying and removing particulates that are worn and particulatesthat have been broken where the diameters are outside of said ranges ofparticle diameters of the large-diameter particles and thesmall-diameter particles, separating the residual shot into thelarge-diameter particles and the small-diameter particles, separatelytransferring to respective flow-control valves the large-diameterparticles and the small-diameter particles, combining again theseparately transferred large-diameter particles and small-diameterparticles in the predetermined weight ratio under control of therespective flow control valves, and reprojecting the recycled, residual,combined particulate shot upon the surface of the product.
 6. A methodfor shot-peening the surface of a product, comprising combining in apredetermined weight ratio large-diameter particles having an averageparticle diameter in the range of 500 to 1,000 μm, medium-diameterparticles having an average particle diameter in the range of 100 to 500μm, and small-diameter particles having an average particle diameter inthe range of 20 to 100 μm to form combined particulate shot, providing aratio of the average particle diameter of said medium-diameter particlesto the average particle diameter of said large-diameter particles and aratio of the average particle diameter of said small-diameter particlesto the average particle diameter of said medium-diameter particles bothin the range of 1/3 to 1/15, shot-peening the surface of the product byprojecting the combined particulate shot upon the surface of the productand obtaining during the shot-peening process by said predeterminationof the weight ratio an area of coverage of the surface of the product byeach of the large-diameter particles, the medium-diameter particles andthe small-diameter particles of at least 100%, said shot-peeninggenerating compressive residual stress at the surface of the product andin a deep layer below the surface of the product.
 7. The method forshot-peening as claimed in claim 6, further comprising recycling thecombined particulate shot after the shot-peening step, includingremoving particulates that are worn and particulates that have beenbroken whose diameters are outside of said ranges of particle diametersof the large-diameter particles, the medium-diameter particles, and thesmall-diameter particles, separating the residual shot into thelarge-diameter particles, the medium-diameter particles, and thesmall-diameter particles, combining again the large-diameter particles,the medium-diameter particles, and the small-diameter particles to formcombined particulate shot, uniformly mixing the combined particulateshot, and projecting again the combined particulate shot upon thesurface of the product.
 8. The method for shot-peening as claimed inclaim 6, further comprising recycling the combined particulate shotafter the shot-peening step and removing particulates that are worn andparticulates that have been broken where the diameters are outside ofsaid ranges of particle diameters of the large-diameter particles, themedium-diameter particles, and the small-diameter particles, separatingthe residual shot into the large-diameter particles, the medium-diameterparticles, and the small-diameter particles, separately transferring torespective flow control valves the large-diameter particles, themedium-diameter particles, and the small-diameter particles, combiningagain the separately transferred large-diameter particles,medium-diameter particles, and small-diameter particles in thepredetermined weight ratio under control of the respective flow controlvalves, and reprojecting the recycled, residual, combined particulateshot upon the surface of the product.
 9. A device for a shot-peening thesurface of a product with shot having large-diameter and small-diameterparticles, comprising a peening chamber having at its top a projectingdevice for shot, a shot-classifying device connected to the bottom ofthe peening chamber for classifying and removing shot that is worn orbroken into particles having particle diameters outside of preselectedrespective ranges of average particle diameters for the large-diameterparticles and small-diameter particles a n d for dividing the residualshot into large-diameter particles and small-diameter particles havingthe aforesaid preselected respective ranges of average particlediameters, discharge openings on the shot-classifying device fordischarging respectively the large-diameter particles and thesmall-diameter particles, flow control valves, conveyors communicatingwith the discharge openings for transferring respectively the dischargedlarge-diameter particles and small-diameter particles to respective flowcontrol valves, and supply pipes connected between the flow controlvalves and the projecting device to provide respectively thelarge-diameter particles and the small-diameter particles to theprojecting device.